Question

why is mercury not the hottest planet in the solar system

Answer 1

Mercury is the hottest planet because it was the nearest planet to sun

Answer 2

Mercury is the planet that is closest to the sun and therefore gets more direct heat, but even it isn't the hottest . Venus is the second planet from the sun and has a temperature that is maintained at 462 degrees Celsius, no matter where you go on the planet. It is the hottest planet in the solar system.